You can show immunity on interrupts on target and focus.
It would be really cool if you could see your own immunity on interrupts on the player castbar.

I think the necessary steps are (also includes pet-castbar):

Quartz

Player.lua
line 99:
--options.args.noInterruptGroup = nil

Pet.lua
line 63:
--options.args.noInterruptGroup = nil

CastbarTemplate.lua
line 165:
--if self.unit == "player" and not init then return end